Operational Aspects of Formation Pressure Measurements While Drilling

Abstract

The emergence of formation pressure measurements in the Logging-While-Drilling (LWD) market has highlighted the challenge of analyzing and interpreting the pressures acquired. The dynamic condition of the borehole and the freshly built mudcake add new variables to the problem and contribute in making the measurement more complex to interpret compared with measurements made by conventional wireline-conveyed methods. This paper presents several formation-pressure-whiledrilling case studies conducted in the North Sea. Formation pressure measurements were repeatedly performed under different well conditions and at different times after the bit penetrated the formation. The impact on the measurement of various parameters is analyzed, most notably, the time after drilling and changes in the measurement sequence. The results obtained with the formation pressure while drilling tool are compared to measurements performed with conventional wireline formation testers (WFTs) conducted at the same depths and at various times after drilling. Analysis of formation cores taken in the same intervals as the pressure tests were performed allows a better understanding of the while drilling-derived mobility measurements. Suggestions are made on how to improve the quality and accuracy of the measurements. Introduction Probe-type formation testers measure the pressure at the wellbore wall, the sandface, which, in oversimplified terms, is also the interface between the external mudcake and the formation. Fig. 1 is a sketch of this simplified view. Whether or not the pressure at the sandface is a good estimate of the true, far-field formation pressure depends on both the properties of the mud and the formation. If the filter cake is totally ineffective the formation tester will measure the wellbore pressure, whereas if the mudcake is perfectly sealing, given sufficient time, the tester should measure the true formation pressure. We are concerned here with the situation in which the mudcake forms a less than perfect seal allowing mud filtrate to leak into the formation resulting in a pressure drop from the sandface to the outer reaches of the formation. The radial change in pressure is primarily dependent on the sealing efficiency of the mudcake, the formation mobility and the difference between the wellbore and formation pressures. When the difference in pressure between the sandface and the formation pressure becomes significant, where the measure of significance depends on the application, the formation tester pressure is usually said to be supercharged. The degree to which the formation is supercharged is characterized by the overpressure, ?pS=psf-pf, where psf denotes the sandface pressure and pf represents the far-field formation pressure, (Fig. 1).1,2 Depending on the mud type and the reservoir fluids present, rock wettability and capillary pressure effects may also affect the sandface pressure measurement. It is not difficult to understand that the degree of supercharging depends at least on the history of the filtration rate from the moment the formation was first drilled, with the most recent history having the greatest effect, and, on the ability of the formation to accommodate the influx of filtrate, i.e., the degree of supercharging should be inversely related to the formation (total) mobility.2 Anything that inhibits the ability of the mudcake to seal the formation against filtration increases the supercharging effect; in particular, any action that limits the growth of mudcake or promotes the erosion of an established mudcake, such as, the scraping action of the bit and stabilizer blades and the mud circulation rate. It is known that even after a mudcake has had sufficient time to build to maximum resistance the ability to seal against invasion can be compromised by wiper trips.3

Focused Sampling of Reservoir Fluids Achieves Undetectable Levels of Contamination

“…• Leaky or incomplete/damaged mudcake, whereby dynamic filtration is still occurring from the wellbore into the formation during sampling (Pop et al 2005).…”

Formation Pressure While Drilling Technology Applied in Arctic Environment: Yamal Region Case Study

“…In the pumps-on mode we have the Important advantage of continuous communication with the tool as well as continuous mud flow to reduce the possibility of sticking. In the end, and in line with the vast majority of FPWD jobs world-wide, (Pop, 2005) the decision was taken to run the service in pumps-on mode. …”

“…Formation Pressure While Drilling tools were introduced to the industry several years ago (Pop, 2005). They have had widespread application and success especially in difficult offshore environemnts (Mishra, 2007) and very long horizontal sections (Larsen, 2008) but until recently they have been limited to high cost, deepwater environments.…”

Demonstrating the Value of Integrating FPWD Measurements With Managed Pressure Drilling To Safely Drill Narrow Mud Weight Windows in HP/HT Environment

“…Continuous invasion and secondary supercharging from mud circulation during the pretest -The action of mud circulation can shear mudcake from the wellbore wall and allow for re-invasion and supercharging of the formation as the pretest is taking place 4 . For drilling applications, such as SBD-2, where the pretests will be taken a short time after drilling the formation, the mudcake will not have much time to develop and become competent to isolate the formation from the wellbore.…”